Verification DM / Handle concurrent .start after .ready

From riotX-android created by [BillCarsonFr](https://github.com/BillCarsonFr): vector-im/riotX-android#794

After the `m.key.verification.ready` event is sent, either party can send an
`m.key.verification.start` event to begin the verification. If both parties
send an `m.key.verification.start` event, and they both specify the same
verification method, then the event sent by the user whose user ID is the
smallest is used, and the other `m.key.verification.start` event is ignored.
In the case of a single user verifying two of their devices, the device ID is
compared instead. If both parties send an `m.key.verification.start` event,
but they specify different verification methods, the verification should be
cancelled with a `code` of `m.unexpected_message`.